A variety of printing methods such as lithographic printing (including normal lithography which uses a dampening solution and waterless lithography which uses no dampening solution), relief printing, intaglio printing and stencil printing have conventionally been used to obtain all manner of printed matter, including printed items on cardboard and various types of books, printed packaging materials such as business form printed items, various plastic printed items, printed items for stickers and labels, and metal printed items. In order to produce this printed matter, inks best suited to each of these printing methods are used. Particularly in the production of cartons typified by cardboard items, various types of printed books, and printed items for stickers and labels, a coating varnish is frequently applied following printing of the ink for purposes including improving the quality of the printed matter and protecting the printed design.
Known examples of such coating varnishes include active energy ray-curable coating varnishes. An active energy ray-curable coating varnish contains an unsaturated compound such as an acrylate ester compound that exhibits curability upon irradiation with active energy rays as a structural component. This type of coating varnish cures instantaneously upon irradiation with active energy rays, with a three dimensional crosslinking reaction of the above unsaturated compound forming a tough coating film. Further, because of this favorable curability, post processing of the coating varnish can be performed immediately following printing. As a result, active energy ray-curable coating varnishes tend to be used widely in the field of printing for packaging, which requires good rub resistance in order to improve productivity and protect the design aesthetics.
In the aforementioned field of printing for packaging, printed matter for foodstuff packaging represents a large proportion of such printed matter. In this particular field, in addition to improvements in the print quality relating to the design aesthetics of the packaging, recent enhanced awareness of safety issues has resulted in a tendency for increased discussion relating to the migration of raw materials from the coating composition into the food contents. This tendency for increased discussion of the migration of coating composition raw materials has naturally tended to be more pronounced in the European and American markets where safety awareness has generally been at a higher level. Switzerland has recently enacted the world's first legal regulation relating to printing inks or coating varnishes for foodstuff packaging (Swiss Ordinance (RS817.023.21), and the effect of this ordinance has spread widely, not only throughout the whole of Europe, but also within other developed nations such as Japan, and other developing nations.
In this ordinance, substances having a certain degree of toxicity or for which concern has been reported in relation to environmental hormones are prohibited for use as raw materials in coating compositions such as printing inks and coating varnishes. Further, the amount of migration permitted into the food contents of the various raw materials used as coating composition raw materials is also strictly limited. The amounts of permitted migration are extremely small, in the order of ppb, making the design of inks extremely demanding. Moreover, large foodstuff manufacturers and foodstuff packaging manufacturers across the entire global market have now started adopting regulations of a similar standard to Swiss Ordinance (RS817.023.21) as their own internal regulations.
In this manner, superior low migration characteristics are now being demanded across the board of raw materials for coating compositions used in foodstuff packaging. In the case of coating varnishes, because the thickness of the coating film on the printed matter is greater than that of an ink, and because the varnish is often used for forming the outermost surface layer on the printed matter, it could be said that the risks associated with outward migration of the raw materials from the coating varnish are particularly large.
As typified by the technology in Patent Documents 1, 2 and 3, research is being conducted from various viewpoints with the aim of achieving low migration, low odor or low bleeding characteristics. Patent Document 1 discloses a method in which by performing chemical bonding between a photopolymerization initiator or a sensitizer and a polyester resin, migration, elution or evaporation of the photopolymerization initiator or the sensitizer following the curing reaction can be reduced. Patent Document 2 discloses a method for reducing odor and migration by appropriate selection of the photopolymerization initiator or sensitizer in an active energy ray-curable ink. Patent Document 3 discloses a method of achieving a combination of superior curability and low bleeding characteristics by using a novel photopolymerization initiator based on a thioxanthone backbone as a sensitizer.
However, the test conditions relating to the evaluations of the low migration characteristics, low odor and low bleeding characteristics are analyzed as below. In Patent Document 1, a film substrate is placed on the printed item, compression is performed at a temperature of 70° C. and a pressure of 100 kgf/cm2 for 5 hours, and the amounts of unreacted initiator and sensitizer that have migrated into the film are measured using a UV-VIS absorption spectrophotometer. Similarly, in Patent Document 2, a film substrate is placed on the printed item, and following standing for 24 hours at a temperature of 60° C. under a pressure of 100 kg/cm2, the film is then immersed in distilled water (temperature: 25° C., 24 hours), and the amounts of unreacted initiator and sensitizer, and unreacted monomers or oligomers, contained within the distilled water are measured using a UV-VIS absorption spectrophotometer. Further, in Patent Document 3, an unprinted white sheet of paper is placed on the printed item, and following standing for 24 hours at a temperature of 60° C. under a pressure of 15 g/cm2, the amounts of the initiator and sensitizer that have migrated onto the white paper are measured.
As described above, Patent Documents 1 and 3 use measurements of the ultraviolet absorption spectrum on a film or sheet of paper, and therefore quantitative measurements at the ppb order are difficult. Moreover, in relation to the measurements in Patent Document 3, the compression conditions and the standing time are comparatively slight. The methods disclosed in Patent Documents 1 and 3 relate mainly to reducing the migration of the photopolymerization initiator and the sensitizer, and make no mention of reducing the migration of the radical polymerizable monomer or oligomer, which is the other main component in the crosslinking reaction.
Further, in Patent Document 2, the film into which migration of the ink raw materials from the printed item is assumed to have occurred is extracted using distilled water. However, in most regulations including Swiss Ordinance (RS817.023.21), the use of a high-concentration alcohol aqueous solution is deemed the most suitable for replicating the actual contents of foodstuff packaging. Furthermore, the temperature and standing time conditions used cannot be said to fully satisfy market demands. In addition, the majority of radical polymerizable monomers or oligomers used in active energy ray-curable inks lack a light absorption wavelength, meaning detection of these compounds using the type of UV-VIS absorption spectrophotometer described in Patent Document 2 is impossible.
In this manner, as enhanced awareness of safety issues relating to foodstuff and pharmaceutical packaging spreads around the world, there are growing demands for active energy ray-curable ink compositions and coating varnishes that satisfy the standards of regulations typified by Swiss Ordinance (RS817.023.21). However, when production of actual products and storage conditions thereof, and further the establishment of appropriate product test conditions were considered, the above regulations are yet to be not satisfied.
Increasing the molecular weight of coating composition raw material compounds is a widely used technique for achieving low migration, low odor and low bleeding characteristics for the raw materials. However, in those cases where a (meth)acrylate monomer is used as the main raw material of an ink or a coating varnish, because increasing the molecular weight of the compound is accompanied by an increase in the molecular weight per (meth)acrylic equivalent in the molecule, the polymerization reactivity tends to deteriorate. On the other hand, with regards the (meth)acrylate monomer within an active energy ray-curable ink or coating varnish, increasing the number of (meth)acrylic groups can improve the curability. However, if the crosslinking density upon curing is too large, then a problem arises in that the cracking resistance of the coating film tends to deteriorate. Further, if an active energy ray-curable coating varnish is used to form the outermost layer on a printed item, then from the viewpoints of enabling the execution of post processing steps following printing of the package, such as die cutting, line marking and foil stamping, it is important that the coating film of the coating varnish exhibits appropriate slipperiness for use in a mechanical line (line applicability).
Accordingly, in the field of printed matter for foodstuff packaging, it is necessary to achieve a combination of meeting the safety standards demanded by the market environment, by suppressing the migration of the coating composition raw materials into the food contents, and improving the production efficiency of the printed matter, by ensuring superior curability and imparting antistatic properties.